Otolith-based approaches indicate strong effects of environmental variation on growth of a Critically Endangered estuarine fish

Abstract

In estuaries, fluctuating environmental conditions exact strenuous physiological demands on the fishes that inhabit these oft-impacted areas, including the Critically Endangered delta smelt Hypomesus transpacificus in California (USA). Using an archive of otoliths spanning 2011-2019, we examined how growth rates of wild subadult delta smelt vary ontogenetically, regionally, and in relation to variation in the physical environment during late-summer and fall in the upper San Francisco Estuary. Recent growth rates were quantified using otolith increment analysis and modeled as functions of both intrinsic (age) and extrinsic (temperature, salinity, clarity, and region) factors using a suite of generalized additive models. Age explained 60% of the variation in log10-transformed growth rates, which peaked at 50-80 d post hatch. Overall, age-adjusted growth rates declined at temperatures >20°C, increased with practical salinity values of 0-4, and exhibited interactive patterns with water clarity. Growth rates appeared highest in the West and Central Delta, and lowest in the North Delta, also corresponding with patterns in environmental conditions. Here, we provide new evidence for how vital rates of wild delta smelt vary spatially and in relation to abiotic environmental variation. Such otolith-based growth reconstructions often provide the first direct look at how the vital rates of wild fish respond to environmental variation in situ, and how future changes are likely to affect the dynamics of wild populations.

Polygenic discrimination of migratory phenotypes in an estuarine forage fish

“…For example, variation in freshwater outflow is known to affect dispersal, habitat suitability, and population dynamics of several species ( Jassby et al 1995 ; Kimmerer 2002 ). Laboratory studies ( Swanson et al 2000 ; Feyrer et al 2007 ; Nobriga et al 2008 ; Komoroske et al 2014 , 2015 ; Jeffries et al 2016 ) and observations of the wild population of Delta Smelt ( Lewis et al 2021 ; Hammock et al 2022 ) indicate strong sensitivity to variation in temperature. In addition to longitudinal studies of life history traits, studies examining the degree to which genotypic and phenotypic variation corresponds with variation in the sensitivity of Delta Smelt to environmental variation remains an unexplored direction of inquiry ( Lewis et al 2022 ).…”

“…However, warming temperatures, hydrological alterations, and habitat destruction have dramatically altered Delta Smelt habitats [ Cloern et al 2011 ; San Francisco Estuary Institute-Aquatic Science Center (SFEI-ASC) 2014 ; Hobbs et al 2017 ; Hutton et al 2017 ]. Contemporary FWR Delta Smelt are now largely relegated to the Sacramento Deep Water Ship Channel in the northern region of the Sacramento-San Joaquin Delta and are abundant only in the coolest of years; whereas MIG Delta Smelt are found more widely throughout the SFE and are more abundant in years with average and above average temperatures ( Hobbs et al 2019 ; Lewis et al 2022 ). The complex life history of Delta Smelt is likely a result of pre-existing adaptation to a wide variety of habitats historically available in the Sacramento-San Joaquin Delta ( Hobbs et al 2019 ), and its ancient origin constitutes an important aspect of the species’ biology that is critical for informing the long-term management of the species.…”

Temperature and salinity preferences of endangered Delta Smelt (Hypomesus transpacificus, Actinopterygii, Osmeridae)

“…Climate change is expected to exacerbate these effects 16 , with projected increases in temperature and salinity exerting direct effects on physiological and fitness parameters of Delta Smelt and increasing extinction risk (i.e., energy allocation, growth, reproduction) 9 , 10 , 17 . For example, elevated water temperatures are associated with declines in otolith increment and condition factor of Delta Smelt collected from the wild 18 , 19 .…”

Visual, spectral, and microchemical quantification of crystalline anomalies in otoliths of wild and cultured delta smelt

“…The delta smelt (Hypomesus transpacificus) is an estuarine forage fish that is endemic to the San Francisco Estuary (SFE). This migratory zooplanktivore is adapted to cool, turbid, low-salinity habitats 33,34 and exhibits a complex life history that allows it to exist within the dynamic low-salinity habitats of the SFE 35 . Although historically abundant, its population has declined to < 1% of 1980s levels, leading to its listing as threatened by the federal Endangered Species Act (ESA) 36 , endangered by the state ESA 37 , and critically endangered by the International Union for Conservation of Nature 38 .…”

“…For example, little is known about the comparative health and survival of cultured versus wild individuals, and such differences could influence the outcomes of experiments and supplementation efforts using cultured fish. Similarly, otolith-based analyses have been used to describe environmental effects on growth 33 and life history 35,43,44 of wild delta smelt; however, the prevalence and potential effects of crystalline anomalies have not been examined. Studies that quantify developmental abnormalities in otoliths, therefore, are important for examining differences in health and fitness among wild and cultured fish populations and for validating the interpretation of otolith-based metrics.…”